Boron nitride (BN) exists in three crystalline forms: hexagonal boron nitride (hBN), a soft form similar to graphite; cubic boron nitride, a hard zincblende form similar to cubic diamond; and wurtzitic boron nitride (wBN), a hard hexagonal form similar to hexagonal diamond. Hexagonal boron nitride is the softest and most stable of the boron nitride crystalline structures, and is routinely used as a lubricant. Cubic boron nitride has a hardness second only to diamond, and therefore has wide application in machining, grinding, drilling and polishing fields. Moreover, cBN rather than diamond is often preferred when working with ferrous materials, as iron catalyzes the decomposition of diamond at elevated temperatures and carbon can change the phase of many iron alloys. The thermal and chemical stability of cBN is also superior to diamond.
Methods for forming hBN and cBN are known in the art. For example, known methods for forming hBN typically involve heating under a protective atmosphere, e.g. in a nitrogen flow, amorphous boron nitride at temperatures above 1500° C. Additionally, known methods for forming cBN typically involves subjecting hBN to similarly high temperatures (e.g. temperatures above 1200° C.) and concurrent high pressures (e.g. pressures above 2 GPa), often in the presence of one or more catalysts or fluxing agents.
As mentioned above, the hardness and chemical and thermal stability of cBN, makes cBN well suited for use as abrasive particles in cutting, grinding, polishing, and drilling media (e.g. tool inserts, twist drills, circular saws, grinding wheels, lapping belts, polishing pads, cutting tools, etc.). Further, cBN monocrystalline particles (e.g. single crystals of cBN) may be bonded together to form a cBN compact, also known as polycrystalline cBN (PCBN). Some or all of the single cBN crystals in a cBN compact may be self-bonded, bonded together with the aid of a bonding medium, or a combination thereof. Suitable bonding media may generally include a metal such as aluminum, cobalt, iron, nickel, platinum, titanium, chromium, tantalum, etc. or an alloy or mixture thereof. Further, a cBN compact may be bonded to a substrate material, such as cemented tungsten carbide, cemented titanium carbide, cemented tantalum carbide, etc.
While cBN and PCBN are widely used to machine materials such as cast iron, powder metal components and other similar materials that are difficult to machine, the cost to fabricate pure cBN and PCBN may be cost prohibitive. For example, the fabrication of both pure cBN and PCBN traditionally requires high temperatures and high pressures. Consequently, metal bonded, polymer bonded, and ceramic bonded cBN components have emerged.
Metal, polymer and ceramic bonded cBN components are generally implemented as grinding and polishing media, typically as coatings on a backing layer. However, polymer bonded cBN components generally suffer from low operating temperatures, and therefore are only capable of providing low material removal rates. Metal bonded cBN components may be capable of higher operating temperatures and therefore higher material removal rates, yet suffer from potentially damaging contamination from the metal binder.